U.S. patent number 5,444,564 [Application Number 08/195,662] was granted by the patent office on 1995-08-22 for optoelectronic controlled rf matching circuit.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to Irwin L. Newberg.
United States Patent |
5,444,564 |
Newberg |
August 22, 1995 |
Optoelectronic controlled RF matching circuit
Abstract
A photonic RF impedance matching system that includes an RF
photonically controlled impedance matching circuit having
adjustable impedance and power transfer characteristics, and
feedback control circuitry for optically controlling the impedance
matching circuit.
Inventors: |
Newberg; Irwin L. (Northridge,
CA) |
Assignee: |
Hughes Aircraft Company (Los
Angeles, CA)
|
Family
ID: |
22722251 |
Appl.
No.: |
08/195,662 |
Filed: |
February 9, 1994 |
Current U.S.
Class: |
398/195; 333/32;
333/17.3 |
Current CPC
Class: |
H03H
7/40 (20130101); H01S 5/06832 (20130101); H01S
5/0427 (20130101) |
Current International
Class: |
H03H
7/40 (20060101); H03H 7/38 (20060101); H01S
5/00 (20060101); H01S 5/0683 (20060101); H04B
010/04 () |
Field of
Search: |
;359/187,161,111,173
;333/32 ;342/54 ;343/860 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Moore; David K.
Assistant Examiner: Negash; Kinfe-Michael
Attorney, Agent or Firm: Alkov; Leonard A. Denson-Low; W.
K.
Claims
What is claimed is:
1. An RF photonic impedance matching system for impedance matching
the input of an RF circuit which provides an RF circuit electrical
output, comprising:
an RF photonically controlled impedance matching circuit having
adjustable impedance and power transfer characteristics responsive
to an RF electrical input for providing an RF matching circuit
output to the input of the RF circuit;
input detecting means for detecting said RF electrical input to
provide an input reference signal;
output detecting means for detecting the RF circuit electrical
output of the RF circuit to provide an output reference signal;
optical signal generating means for providing an optical signal to
control said RF photonically controlled impedance matching circuit;
and
control means responsive to said input reference signal and said
output reference signal for controlling said optical signal
generating means such that said impedance matching circuit provides
a desired impedance matching and power transfer relative to the RF
circuit.
2. The RF photonic impedance matching circuit of claim 1 wherein
said optical signal generating means comprises a laser diode.
3. An RF photonic impedance matching system for matching an
electrical input of an RF optical circuit which provides an RF
optical output, comprising:
an RF photonically controlled impedance matching circuit having
adjustable impedance and power transfer characteristics responsive
to an RF electrical input for providing an RF matching circuit
electrical output to the electrical input of the RF optical
circuit;
input detecting means for detecting said RF electrical input and
providing an input reference signal;
output detecting means for detecting the RF optical output of the
RF optical circuit for providing an output reference signal;
optical signal generating means for providing an optical matching
circuit control signal to control said RF photonically controlled
impedance matching circuit; and
control means responsive to said input reference signal and said
output reference signal for controlling said optical signal means
such that said impedance matching circuit provides a desired
impedance matching and power transfer relative to the RF optical
circuit.
4. The RF photonic impedance matching circuit of claim 3 wherein
said optical signal generating means includes a laser diode.
5. The RF photonic impedance matching circuit of claim 3 wherein
said optical signal generating means includes an optical coupler
responsive to the RF optical output of the RF optical circuit for
providing an optical signal sample of the optical output of the RF
optical circuit, and an optical control circuit responsive to said
control means for controlling said optical signal sample to provide
a controlled optical signal sample which is provided as said
optical matching circuit control signal.
6. An RF photonic impedance matching system for impedance matching
the output of an RF optical circuit which receives an RF optical
input and provides an RF electrical output, comprising:
an RF photonically controlled impedance matching circuit having
adjustable impedance and power transfer characteristics responsive
to the RF electrical output of the RF optical circuit for providing
a matching circuit RF electrical output;
input detecting means for detecting the RF optical input to the RF
optical circuit and providing an input reference signal;
output detecting means for detecting said matching circuit RF
electrical output for providing an output reference signal;
optical signal generating means for providing an matching circuit
optical control signal to control said RF photonically controlled
impedance matching circuit; and
control means responsive to said input reference signal and said
output reference signal for controlling ling said optical signal
generating means such that said impedance matching circuit provides
a desired impedance matching and power transfer relative to the RF
optical circuit.
Description
BACKGROUND OF THE INVENTION
The subject invention is directed to fiber optic links for radar
systems, and is directed more particularly to light controlled
impedance matching techniques for microwave systems using
optoelectronic feedback and control techniques.
In order to fully utilize the potential of advanced microwave
systems with their vast data collection and processing
capabilities, fast and very efficient data communication transfer
systems must be utilized. RF fiber optic links utilizing
optoelectronic components have the capability of meeting such
communications requirements for several reasons. Fiber optic links
are not susceptible to radio frequency noise and are capable of
operating at very high data rates and at high radio frequencies
without creating opportunities for interception and detection. The
light weight and small size of fiber optic cables enables the use
of highly redundant paths between units and provides for easier
mechanical routing in corporate feeds of electronically scanned
arrays, thus improving reliability and damage tolerance. Finally,
the EMI immunity of fiber optics reduces equipment failures caused
by electrical power transients and could also improve the
capabilities and reduce the size and weight of radar and other
avionic systems.
Typically, a fiber optic link includes a photonic transmitter
responsive to an input RF signal for producing an RF amplitude
modulated light signal. The modulated light signal is communicated
via a fiber optic cable to a photonic photodiode detector receiver
that produces an electrical RF output in response to the received
modulated light.
A consideration with present fiber optic links is the typically
high level of insertion loss introduced by a fiber optic link. A
significant factor in the high level of insertion loss is the
typical technique of providing broadband 50 ohm impedance matching
at the RF input and RF output of the fiber optic link for
compatibility with microwave circuits, usually by the addition of
resistance. While such impedance matching provides for reduced
reflections over a relatively broad bandwidth, it results in
substantial power loss.
A further consideration with fiber optic links in general is the
lack of consistency in the impedance characteristics of laser
diodes utilized in photonic transmitters. There is also a lack of
consistency in the impedance characteristics of the photodiodes
utilized in photonic receivers, although to a lesser degree.
SUMMARY OF THE INVENTION
It would therefore be an advantage to provide an RF fiber optic
link that provides for photonically controlled impedance
matching.
Another advantage would be to provide an RF fiber optic link that
provides for increased power transfer with photonically adjustable
impedance matching circuitry.
A further advantage would be to provide a photonically controlled
RF impedance matching technique that reduces insertion loss of
fiber optic links.
The foregoing and other advantages are provided by the invention in
a photonic RF impedance matching system that includes an RF
photonically controlled impedance matching circuit having
adjustable impedance and power transfer characteristics, and
feedback control circuitry for optically controlling the impedance
matching circuit.
BRIEF DESCRIPTION OF THE DRAWING
The advantages and features of the disclosed invention will readily
be appreciated by persons skilled in the art from the following
detailed description when read in conjunction with the drawing
wherein:
FIG. 1 is a schematic block diagram of an example of a photonic
impedance matching system in accordance with the invention for
matching the input of a laser diode circuit.
FIG. 2 is a schematic block diagram of a further example of a
photonic impedance matching system in accordance with the invention
for matching the input of a laser diode circuit.
FIG. 3 is a schematic block diagram of an example of a photonic
impedance matching system in accordance with the invention for
matching the input of an RF circuit.
FIG. 4 is a schematic block diagram of an example of a photonic
impedance matching system in accordance with the invention for
matching the output of an optical detector circuit.
FIG. 5 is a simplified schematic diagram of a photonically
controlled variable length transmission line that can be used for
sections of a photonically controlled stub tuner utilized as a
photonically controlled impedance matching circuit in the photonic
impedance matching system of the invention.
DETAILED DESCRIPTION
In the following detailed description and in the several figures of
the drawing, like elements are identified with like numerals.
The subject invention is generally directed to photonic RF feedback
impedance matching circuitry for RF optical and electrical
circuits. In accordance with the invention, a feedback signal,
which can be electrical or optical, is derived from the output of
the RF circuit being input matched or from the output of a
photonically controlled impedance matching circuit in the case of
output matching, and is processed with an input signal to the
circuit being matched to determine a optical correction signal that
controls a controllable impedance matching circuit that includes
components responsive to the optical correction signal.
Referring now to FIG. 1, schematically depicted therein is an
impedance matching system in accordance with the invention for
matching the input of a laser diode circuit 13 which provides an
optical output signal that is coupled to a fiber optic cable 15.
For example, the laser diode circuit 13 comprises a transmitter for
a fiber optic link. The impedance matching system includes an
optoelectronic RF matching circuit 11 that receives an RF input
signal on an RF input line 12 from an RF source (not shown) and
transfers the RF input signal to the laser diode circuit 13. The RF
matching circuit 11 and the laser diode circuit 13 are implemented
on the same substrate so as to minimize the distance between the
matching circuit and the laser diode circuit for which matching is
being provided.
A first optical coupler 17 on the fiber optic cable 15 provides a
sample of the optical output of the laser diode circuit 13 to an RF
photodiode detector 19 which provides a variable output reference
signal indicative of RF power of the optical output of the laser
diode circuit 13. The output of the RF photodiode detector 19 is
provided to a comparator circuit 21 which also receives an
electrical reference signal REF as an input. An RF coupler 23 on
the RF input line 12 to the matching circuit 11 provides a sample
of the RF input signal to an RF detector 25 whose output comprises
a variable input reference signal indicative of the RF power of the
RF electrical signal input to the laser diode circuit 13. The
variable input reference signal provided by the RF detector 25 is
provided as another input to the comparator circuit 21.
In accordance with known techniques, the comparator circuit
normalizes the output reference signal relative to the input
reference signal, and compares such normalized output reference
signal to the reference signal REF which is of the desired
normalized power output, such as the maximum normalized power
output. Pursuant to such comparison, the comparator circuit
produces a control signal that is provided to an optical control
circuit 27 which receives a sample of the laser diode circuit
output from a second optical coupler 14 on the fiber optic cable
15. The optical control circuit 27 produces a light control signal
output which varies in response to the control signal output of the
comparator circuit 21. The light control signal output of the
optical control circuit 27 is provided via a fiber optic cable 29
to the optoelectronic matching circuit 151 which changes its
impedance in response to the light control signal provided
thereto.
In operation, the optoelectronic matching circuit 11 and related
components comprise a broad bandwidth microwave impedance matching
system that provides input impedance matching for the laser diode
circuit. Lasers are very broad bandwidth, typically from DC to
about 10 GHz, and thus very broad bandwidth impedance matching is
desirable. Impedance matching is achieved by using a feedback
technique to correct the input match via light control of the
optoelectronic RF matching circuit 11 which changes impedance in
response to the light control signal. The light control signal is
obtained by coupling a light sample from the laser diode circuit
light output to the RF photodiode detector that provides the
variable output reference signal which comprises an electrical
sample of the RF signal modulated onto the laser light output. This
variable output reference signal is normalized relative to the
variable input reference signal which comprises an electrical
sample of the RF input signal to the laser diode circuit and is
provided by the RF coupler and RF detector. The normalized output
reference signal is compared with the electrical reference signal
REF used to establish the predicted maximum power output level for
the general class of laser diodes as utilized in the laser diode
circuit. The control signal provided by the comparator circuit (a
general type circuit known in the art) controls the optical circuit
which can be an external modulator or any type of optoelectric
circuit that can control an input light level amplitude via an
electrical input. The optical control circuit receives its light
input from the second light coupler, and the variable light signal
out of the optical control circuit interacts with the impedance
matching circuit to obtain a broad bandwidth match.
Referring now to FIG. 2, schematically depicted therein is an
impedance matching system in accordance with the invention for
matching the input of a laser diode circuit 53 which provides an
optical output signal that is coupled to a fiber optic cable 55.
For example, the laser diode circuit 53 comprises a transmitter for
a fiber optic link. The impedance matching system includes an
optoelectronic RF matching circuit 51 that receives an RF input
signal on an RF input line 52 from an RF source (not shown) and
transfers the RF input signal to the laser diode circuit 53. The RF
matching circuit 51 and the laser diode circuit 53 are implemented
on the same substrate so as to minimize the distance between the
matching circuit and the laser diode circuit for which matching is
being provided.
An optical coupler 57 on the fiber optic cable 55 provides a sample
of the optical output of the laser diode circuit 53 to an RF
photodiode detector 59 which provides a variable output reference
signal indicative of RF power of the optical output of the laser
diode circuit 53. The output of the RF photodiode detector 59 is
provided to a comparator circuit 61 which also receives an
electrical reference signal REF as an input. An RF coupler 63 on
the RF input line 52 to the matching circuit 51 provides a sample
of the RF input signal to an RF detector 65 which provides a
variable input reference signal indicative of the RF power of the
RF electrical signal input to the laser diode circuit 53. The
variable input reference signal is provided as another input to the
comparator circuit 21.
In accordance with known techniques, the comparator circuit
normalizes the output reference signal relative to the input
reference signal, and compares such normalized output reference
signal to the reference signal REF which is of the desired
normalized power output, such as the maximum normalized power
output. Pursuant to such comparison, the comparator circuit
produces a control signal that is provided to a laser diode circuit
67 which is biased by a DC bias voltage. The laser diode circuit 67
produces a control light output which varies in response to the
control signal output of the comparator circuit 61. The control
light output of the laser diode circuit 67 is provided via a fiber
optic cable 69 to the optoelectronic matching circuit 51 which
changes its impedance in response to the control light provided
thereto.
The impedance matching system of FIG. 2 operates similarly to the
impedance matching system of FIG. 1, except that the control light
for the optoelectronic matching circuit is provided by a laser
diode circuit light output rather than by a modulated sample of the
output of the laser diode circuit whose input is being matched.
Referring now to FIG. 3, schematically depicted therein is an
impedance matching system in accordance with the invention for
matching the input of an RF circuit 113 such as an FET amplifier
which provides an electrical RF output on an RF output line 115.
The impedance matching system includes an optoelectronic RF
matching circuit 111 that receives an RF input signal on an RF
input line 112 from an RF source (not shown) and transfers the RF
input signal to the RF circuit 113. The RF matching circuit 111 and
the RF circuit 113 are implemented on the same substrate so as to
minimize the distance between the matching circuit and the RF
circuit for which matching is being provided.
An RF coupler 117 on the RF output line 115 provides a sample of
the electrical RF output of the RF circuit 113 to an RF detector
119 which provides a variable output reference signal indicative of
the RF power of the electrical output of the RF circuit 113. The
output of the RF detector 59 is provided to a comparator circuit
121 which also receives an electrical reference signal REF as an
input. An RF coupler 123 on the RF input line 112 to the matching
circuit 111 provides a sample of the RF input signal to an RF
detector 125 which provides a variable input reference signal
indicative of the RF power of the RF electrical signal input to the
RF circuit 113. The input reference signal is provided as another
input to the comparator circuit 121.
The comparator circuit 121 provides a control signal to a laser
diode circuit 127 which is biased by a DC bias voltage. The laser
diode circuit 127 produces a control light output which varies in
response to the control signal output of the comparator circuit
121. The control light output of the laser diode circuit 127 is
provided via a fiber optic cable 129 to the optoelectronic matching
circuit 111 which changes impedance in response to the control
light provided thereto.
The feedback impedance matching system of FIG. 3 operates similarly
to the impedance matching system of FIG. 2, except that an
electrical RF circuit is being matched instead of a laser diode
circuit, and thus a sample of an electrical RF output is utilized
as an input to the comparator instead of a sample of an RF
modulated light output.
Referring now to FIG. 4, schematically depicted therein is an
impedance matching system in accordance with the invention for
impedance matching the output of an RF photodiode detector 153
which receives an RF modulated input optical signal via a fiber
optic cable 152. The output impedance matching system includes an
optoelectronic RF matching circuit 151 that receives an RF signal
from the RF photodiode detector 153 and provides RF matching
circuit output signal on an RF output line 155. The RF photodiode
detector 153 and the matching circuit 151 are implemented on the
same substrate so as to minimize the distance between the matching
circuit and the RF photodiode detector for which output matching is
being provided.
An RF coupler 157 on the RF output line 155 provides a sample of
the electrical RF output of the optoelectronic matching circuit 151
to an RF detector 159 which provides a variable output reference
signal indicative of the RF power of the electrical output of the
optoelectronic matching circuit. The output of the RF detector 159
is provided to a comparator circuit 161 which also receives an
electrical reference signal REF as an input. An optical coupler 163
on the optical fiber cable input 152 to the photodiode detector 153
provides a sample of the optical input signal to an RF photodiode
detector 165 which provides an electrical output to a broad
bandwidth 50 ohm load 166 whose output comprises a variable input
reference signal that is indicative of the RF power of the RF
electrical signal input to the RF power of the optical signal input
to the RF photodiode detector. The variable input reference signal
is provided as another input to the comparator circuit 161.
The comparator circuit 161 provides a control signal to a laser
diode circuit 167 which is biased by a DC bias voltage. The laser
diode circuit 167 produces a control light output which varies in
response to the control signal output of the comparator circuit
161. The control light output of the laser diode circuit 167 is
provided via a fiber optic cable 169 to the optoelectronic matching
circuit 151 which changes impedance in response to the control
light provided thereto.
The feedback impedance matching system of FIG. 4 operates similarly
to the impedance matching system of FIG. 3, except that output
matching is being provided and the reference input signal to the
comparator circuit is obtained from an optical coupler on the fiber
optic cable input to the photodetector whose output is being
matched. The optical signal from the optical coupler is transformed
to an RF electrical signal by an RF photodiode detector whose
output is provided to the 50 ohm load which provides to the
comparator circuit a variable input reference signal that comprises
a broadband electrical sample of the RF optical signal input to the
RF photodiode detector whose output is being matched.
In accordance with the invention, the matching circuits comprise
active circuitry whose impedance matching characteristics are
adjusted and controlled by optical signals so as to provide for
impedance matching (i.e., reduced reflections) and increased power
transfer as compared to the conventional 50 ohm broadband impedance
matching. Such circuitry provides the capability of compensating
variations as to design and manufacturing process of the components
being matched, and also the capability of changing the match for
different operating frequencies while providing for increased power
transfer.
The use of active impedance matching circuits having photonically
controllable impedance matching and power transfer characteristics
permits for adjustment of the matching circuits for impedance
matching optimization, power transfer optimization, as well as
changes and comparisons, while maintaining the same RF circuit
configuration. The active matching circuit can be used for input
match or output matching over a desired frequency range centered at
a preselected operating frequency. This covers the condition where
the intended frequency range of use is known and the active match
is used to account for the differences in the impedance parameters
of the circuitry being matched. The active matching could also be
used over a large frequency range by tuning the active match as the
system operating frequency is changed. This covers the case where
it is needed to change the impedance match in the course of
operation in a system where the input frequency changes.
Power transfer is increased between the signal source and the load
when impedance matching is used: (1) to make the reactance of the
load equal and opposite to that of the signal source; and (2) to
make the resistive component of the load equal to the resistive
component of the signal source. This condition is referred to as a
"conjugate match." When the above conditions are met, then maximum
signal source power is transferred to the load. The matching
circuit is typically tuned to the particular operating frequency
since most matching is relatively narrow bandwidth. To the extent
that the matching circuits can be made broadband, the match will be
good over that frequency range. Thus, one parameter or set of
parameters would be to meet the conjugate match condition over as
wide a frequency range as possible. One way to help provide a wide
bandwidth match is to have the signal source as close in distance
as possible to the load. Close in this case would mean less than
one-eighth of a wavelength at the highest operating frequency.
Also, as the frequency is increased, distributed matching elements
should be used rather than lumped elements. The matching devices
and connecting transmission line should have minimum losses. Any
loss in the matching circuit will reduce the power transferred to
the load.
Several different matching networks can be configured by using
inductors and capacitors in combination, including a "Tee" network
wherein the "arms" of the "Tee" are inductors and the center leg is
a capacitor to ground. The capacitor to ground could be a varactor
diode which has a variable capacity that can be photonically
"tuned" to different values. Thus, the varactor is the active
device used to change the impedance match at a given frequency or
range of frequencies. An example of a "Tee" network is shown on
page 20-36 of the "Matching Network Designs with Computer
Solutions" section of the 1980 Motorola RF Data Manual.
A combination of transmission line sections such as single, double
and triple-stub tuners can also be used to match an impedance that
is both resistive and reactive. This is accomplished by changing
the length of these transmission lines. Techniques for matching
transmission line stubs are known in the art, including those
disclosed in pages 193 through 201 of "Transmission Lines and
Networks," Walter C. Johnson, McGraw-Hill Book Company, Inc., 1980;
and the use of a variable length transmission line for impedance
matching is known in the art, as disclosed, for example, in
commonly assigned U.S. Pat. 5,014,023, issued May 7, 1991 to
Mantele for "NON-DISPERSIVE VARIABLE PHASE SHIFTER AND VARIABLE
LENGTH TRANSMISSION LINE," incorporated herein by reference.
Referring now to FIG. 5, set forth therein by way of illustrative
example is a simplified schematic of a variable length distributed
transmission line circuit that can be utilized for the sections of
a combination of transmission line sections utilized as the
impedance matching circuit in the circuits of FIGS. 1 through 4.
The variable length distributed transmission line circuit includes
first and second conductors 51, 53, and plurality of optically
controlled varactor diodes D.sub.v connected across the conductors
51, 53. The distributed parameters of the transmission line are
represented as inductors L.sub.D, and the variable junction
capacitances of the varactor diodes are depicted as variable
capacitors C.sub.D. The electrical length of the transmission line
is varied by controlling the variable capacitances of the varactor
diodes with light. The use of varactor diodes to control a variable
length transmission line is known in the art as disclosed in the
previously cited U.S. Pat. 5,014,023 which discloses voltage
control of varactor diodes to change the length of the transmission
line. Also, the use of light to control the capacitance of a
varactor is also known in the art, as disclosed, for example, in
the paper entitled "A HIGH-SPEED PHASE SHIFTER BASED ON OPTICAL
INJECTION," L. R. Brothers Jr. and C. H. Cox, III, 1987 IEEE MTT-S
Digest, pages 819-823, which is incorporated herein by
reference.
The foregoing has been a disclosure of photonically controlled
reconfigurable RF matching circuitry for RF electrical and photonic
circuits which provide for optimal impedance matching (i.e., a low
level of reflections), reduced insertion loss in comparison to
conventional 50 ohm matching techniques (i.e., increased power
transfer), and the capability for changing impedance and bandwidth
characteristics.
Although the foregoing has been a description and illustration of
specific embodiments of the invention, various modifications and
changes thereto can be made by persons skilled in the art without
departing from the scope and spirit of the invention as defined by
the following claims.
* * * * *